Afocal attachments are used to change the field-of-view and focal length of objective lens systems. Known attachments generally include two or more afocal elements mounted as pairs in a holding device. The holding device is inserted in the optical axis of the objective and can be rotated about an axis to position the lenses in a desired orientation to alter the field of view of the objective. Oriented in one direction through a pair of lenses, such attachments may increase the focal length, and, in the other direction, correspondingly decrease the focal length.
In devices where multiple afocal pairs are used, conventional rotating and positioning systems tend to require complex sliding or pivoting mechanisms and take up considerable space.
The present invention, generally, provides a device for changing the field-of-view of an objective with four conversions that is compact and simple in structure.
More particularly, the present invention provides a converter device for an optical lens system having an optical axis, the device comprising a mounting frame defining six faces in a cuboid arrangement which is disposed in the optical axis between the objective lens system and an object side of the viewing device. The device includes a first lens mounted on a first face of the frame, a second lens mounted on a second face of the frame, the second face being oppositely located and parallel to the first face, the first and second lenses forming a first afocal pair on a first axis. In addition, a third lens is mounted on a third face of the frame, and a fourth lens is mounted on a fourth face of the frame opposite and parallel to the third face, the third and fourth lenses forming a second afocal pair on a second axis perpendicular to the third and fourth refractive lenses.
According to another aspect of the invention, the lens mounted in the frame are refractive lenses.
According to another aspect of the invention, the fifth and sixth faces of the frame are left empty.
The mounting frame in accordance with the present invention is rotatable about a third axis perpendicular to the first and second axes for selectively aligning one of the first afocal pair and the second afocal pair with the optical axis.
The mounting frame is also rotatable about the first or the second axes for rotating both the first and second afocal pairs from the optical axis and positioning the fifth and sixth faces of the frame in the optical axis.
According to another aspect of the invention, the lenses making up each of the afocal pairs are selected to have opposite powers. For example, for each afocal pair, a first lens is a positive lens and the second lens is a negative lens. Each lens pair, therefore, provides a telephoto conversion or a wide angle conversion depending on the direction the pair is oriented.
Thus, the converter according to the invention is capable of providing five conversion conditions which include a zero conversion condition in which no afocal pair is interposed in the optical axis and the power of the system is the unaltered objective lens focal length.